How to make beveled 90 degree corners on a curve better?
First make sure your Bezier Curve type  is set to 2D (if you don't need a 3D curve that is), that will automatically reduce deformation on tight corners.

It is a known limitation of the current system and there is no way around it at time moment. This was planned to be improved as part of 2016 GSOC Bezier Curve Improvements, but never actually made it.

Secondly in edit mode make sure your Spline Type is set to Bezier, it seems to be set to Poly which doesn't allow curved segments.

Thirdly erase the corner vertex and, as Paul Gonet mentioned turn the two contiguous handles to Aligned. If you still need actual 3D curves this technique will also reduce deformation considerably.



Lastly if you need to bevel curves this frequently the Curve Fillet addon from the Blender Market may help and simplify things in the long run. Disclaimer: Its a commercial paid addon, I am not the author nor affiliated with it in any way, though I did purchase it and use it regularly